Carvedilol is a nonselective β-adrenergic blocking agent with α1 blocking activity. Carvedilol, also known as (±) 1-(9H-carbazol-4-yloxy)-3-[[2(2-methoxyphenoxy)ethyl]amino]-2-propanol, (CAS Registry No. 72956-09-3) has the structure of formula I.

Carvedilol has a chiral center and can exist either as individual stereoisomers or in racemic form. Racemic carvedilol is the active ingredient of COREG®, which is indicated for the treatment of congestive heart failure and hypertension. The nonselective β-adrenergic activity of carvedilol is present in the S(−) enantiomer and the α1 blocking activity is present in both the R(+) and S(−) enantiomers at equal potency. Both the racemate and stereoisomers may be obtained according to procedures well known in the art (EP B 0127 099).
Synthesis of Carvedilol
U.S. Pat. No. 4,503,067, which is incorporated herein by reference, discloses a process of preparing carvedilol by the following reaction:
in which 4-(oxiran-2-ylmethoxy)-9H-carbazole (formula II) is reacted with (2-(2-methoxyphenoxy)ethylamine (formula III) to form carvedilol (I). The above process produces a low yield of carvedilol at least in part because in addition to carvedilol, the process leads to the production of a bis impurity of the following structure (formula IV):
(See EP 918055.)
In order to reduce the formation of the formula IV and to increase the yield of carvedilol, EP 918055 discloses using abenzyl protected form of the 2-(2-methoxyphenoxy)ethylamine (III).
Carvedilol Polymorphs
International application No. WO 99/05105, incorporated herein by reference, discloses that carvedilol can be isolated as two polymorphic forms, depending on the method of preparation. The two polymorphic forms, designated Form I and Form II, are reported to be monotropic and are distinguishable by their infrared, Raman and X-ray powder diffraction spectra. No evidence is found in the literature about the existence of hydrated solvate states of carvedilol.
Polymorphism is the property of some molecules and molecular complexes to assume more than one crystalline form in the solid state. A single molecule may give rise to a variety of crystal forms (also called “polymorphs”, “hydrates” or “solvates”) having distinct physical properties. For a general review of polymorphs and the pharmaceutical applications of polymorphs see G. M. Wall, Pharm Manuf. 3, 33 (1986); J. K. Haleblian and W. McCrone, J. Pharm. Sci., 58, 911 (1969); and J. K. Haleblian, J. Pharm. Sci., 64, 1269 (1975), all of which are incorporated herein by reference.
The existence and physical properties of different crystal forms can be determined by a variety of techniques such as X-ray diffraction spectroscopy, differential scanning calorimetry and infrared spectroscopy. Differences in the physical properties of different crystal forms result from the orientation and intermolecular interactions of adjacent molecules (complexes) in the bulk solid. Accordingly, polymorphs, hydrates and solvates are distinct solids sharing the same molecular formula yet having distinct advantageous and/or disadvantageous physical properties compared to other forms in the polymorph family. The existence and physical properties of polymorphs, hydrates and solvates is unpredictable.
One of the most important physical properties of a pharmaceutical compound which can form polymorphs, hydrates or solvates, is its solubility in aqueous solution, particularly the solubility in gastric juices of a patient. Other important properties relate to the ease of processing the form into pharmaceutical dosages, such as the tendency of a powdered or granulated form to flow and the surface properties that determine whether crystals of the form will adhere to each other when compacted into a tablet.